This application claims the priority of Japanese Patent Application No. 3-129949 filed on May 31, 1991 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an actuator, and more particularly, to an actuator which drives, for example, an intake valve in an internal combustion engine.
2. Description of the Related Art
A conventional actuator of this type is disclosed in Japanese Unexamined Patent Publication No. 2-140419. As shown in FIG. 5, such an actuator 30 comprises a four-pole stepping motor 31 whose core 32 is shaped into a square barrel. The core 32 includes a peripheral wall 32a which forms magnetic paths and four poles 33a to 33d which protrude inward from the peripheral wall 32a. The core 32 is formed of a plurality of laminated metal plates. Each of the metal plates is made through a pressing process and has four side edges linked to one another and four projections which extend inward from the respective side edges of the plate in association with the poles 33a to 33d, respectively. The core 32 has coils 34a to 34d wound around the poles 33a to 33d, respectively. A rotor 36 with a magnet 36a is held rotatable in the center of the core 32. The rotor 36 will be intermittently rotated every 90 degrees by properly changing the direction of conduction to one coil pair 34b and 34d while keeping the direction of conduction to the other coil pair 34a and 34c the same.
The prior art described above is designed to have the poles 33a to 33d projecting inward from the inner surface of the peripheral wall 32a. With this structure, to wind the coils 34a through 34d around the respective poles 33a-33d is relatively difficult because the adjacent poles, which have their coils already wound thereabout, and the peripheral wall 32a interfere with this work.
Since the coils 34a to 34d should be arranged so that they do not interfere with one another, clearances have to be provided between the coils. The internal space of the core 32 cannot therefore be effectively used.
The opposing pair of poles 33a and 33c in FIG. 5 may be replaced respectively with magnets 35a and 35b shown in FIG. 6. This provides another type of actuator in which the rotor 36 may be rotated in 90 degree intervals. This actuator has its core 31 provided with a pair of poles 33b and 33d. It is therefore easier to wind the coils 34b and 34d around the poles 33b and 33d in this case than in the above described prior art.
This modified actuator does not differ from the conventional case in that the peripheral wall 32a is still an obstruction when the coils 34b and 34d are to be wound around the poles 33b and 33d. Further, this modification gives rise to a new shortcoming where the magnets 35a and 35b have to be incorporated at the determined positions of the metallic core against their magnetic force.